The present invention relates to a method of adjusting the composition of molten steel in an arc process in which an electrode is submerged in a slag layer on molten steel contained in a ladle, and an electric arc is generated between the molten steel and the electrode so as to arc-heat the molten steel.
Molten steel is refined in a steel converter and an arc process is then performed, thereby decreasing the temperature of the molten steel refined in the converter. Thus, losses of additives used and consumption of furnace wall refractory material of the converter can be decreased, and the yield of the molten steel from the converter can be improved. When the arc process is performed in practice, preferably, a carbon electrode is submerged in a slag layer on the molten steel, and an electric arc is generated between the electrode and the molten steel while the electrode is substantially covered with the slag (namely, submerged arc heating). When the slag does not cover the electrode, an arc jumps between the electrode and the ladle furnace wall or between the electrode and a gas blow lance, thus causing damage to the refractory material in these portions. In addition, the furnace wall refractory material is heated undesirably by heat radiated from an arc column.
When the molten steel is arc-heated, an alloy is added to the molten steel so as to adjust the composition of the molten steel. However, in the case of submerged arc heating, since the slag layer is on the molten steel, the added alloy is prevented by the slag from mixing with the molten steel. For this reason, the efficiency of an alloy additive is low.
In the arc process, when the composition of the molten steel is adjusted, the molten steel is poured from the converter into the ladle, and the oxygen partial pressure P.sub.O.sbsb.2 of the molten steel in the ladle is measured, as shown in the timing chart in FIG. 1. Then, aluminum serving as a deoxidation material is added in the molten steel in accordance with the measured P.sub.O.sbsb.2 value. The molten steel is then arc-heated so as to perform deoxidation. Thereafter, the composition of the molten steel is analyzed by sampling, and the molten steel is arc-heated again. When an analysis result is determined, arc heating is stopped and a desired amount of ferro-alloy is added to the molten steel. Ar gas is blown into the molten steel so as to stir it. Finally, the molten steel is sampled and refining by the arc process thus reaches completion.
However, in the conventional method for adjusting the composition, arc-heating of the molten steel must be continued until the analysis result is determined by sampling the molten steel, which is a time-consuming process. In addition to this, deoxidation by adding Al and adjustment of the composition by adding ferro-alloy are performed at different points, thus further prolonging the time required for processing.
In order to control sulfide inclusion to a spherical shape in solidified steel, calcium (Ca) is added in the molten steel. However, since Ca has a lower boiling point (1,483.degree. C.) than the melting point of the steel and an extremely high vapor pressure, even if a Ca alloy is added to the molten steel by a so-called injection method, Ca-wire method, bullet shooting method or the like, the yield of Ca added is low, e.g., about 10%. In addition, the yield varies widely. For this reason, much of the Ca alloy used is wasted, and hydrogen and nitrogen pick-up of the molten steel and a decrease in a temperature thereof occur.
Furthermore, in order to manufacture high nitrogen steel, manganese nitride is conventionally added to the molten steel. However, when manganese nitride is used, the cost is increased. The yield of manganese nitride is unstable, and magnanese content of the molten steel is increased undesirably.